1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a printer, and more particularly to an image forming apparatus in which a fixed brush charger charges a surface of an image carrier to form a charged area, which is subjected to an image exposure to form an electrostatic latent image. The present invention also relates to a method for forming an image.
2. Description of the Related Art
Electrophotographic image forming apparatuses have employed several types of charging devices such as a corona charger and a brush charger which charge surfaces of image carriers such as photosensitive members to form electrostatic latent images. The brush chargers have attracted the attention, because they generate less ozone, which may deteriorate the surfaces of the photosensitive members and adversely affect the human bodies, as compared with the corona chargers. Particularly, the fixed brush chargers, which are fixed in the moving direction of the surfaces of the photosensitive members, have attracted the more attention, because they are simpler than rotary brush chargers.
However, if the fixed brush charger is used for the photosensitive member having a soft surface such as an organic photosensitive member, the photosensitive member may be shaved, because tip ends of brush fibers of the charger continuously contact the soft surface of the photosensitive member. This shaving generates fine powder having a particle diameter in a range from 0.1 to several microns from the photosensitive member. The fine powder adheres onto the brush fibers to reduce the charging performance, which may cause a noise on the image. That is; a small amount of charge is charged by portions of the brush fibers having a large amount of fine powder adhering thereto, and a large amount of charge is charged by portions of the brush fibers having a small amount of powder adhering thereto. As a result, a striped noise is generated on the image by the fine powder thus generated from the photosensitive member and adhering onto the fibers.
The above problem may be solved by appropriately separating the ends of the brush fibers of the brush charger from the surface of the photosensitive member so as to avoid continuous contact of particular brush fibers with a particular portion of the photosensitive member. For this purpose, following constructions or methods have been proposed. In the construction disclosed in the Japanese Laid-Open Patent Publication No. 59-37568, a plurality of brush chargers are disposed on a rotary support, and the rotary support is intermittently rotated for sequentially using the brushes. In the construction disclosed in the Japanese Laid-Open Patent Publication No. 59-204858, the photosensitive member is provided at its end with a cam, which is used to separate the brush charger from the photosensitive member during the stop of the photosensitive member. In the construction disclosed in the Japanese Laid-Open Patent Publication No. 59-204859, the photosensitive member is provided at its end with a cam touching with a roll on a shaft of a brush charger to operate the brush charger from the photosensitive member during the stop of the photosensitive member. In the construction disclosed in the Japanese Laid-Open Patent publication No. 59-204860, a pulley which cooperates with the rotating photosensitive member operates to contact the brush charger with the photosensitive member.
However, the above-noted constructions practically require large and complicated mechanisms, resulting in increased dimensions of the image forming apparatus. This cannot comply with the demand for the reduced dimensions and compact structures of the image forming apparatus in these days.
Accordingly, it is an object of the invention to provide an image forming apparatus, in which a surface of an image carrier such as a photosensitive member is charged by a brush charger to form a charged area, which is subjected to an image exposure to form an electrostatic latent image, and particularly to prevent adhesion of shaved powder of the image carrier onto brush fibers of the brush charger in a simple manner without remarkably increasing the dimensions of the image forming apparatus for obtaining a good image.